Piezoelectric speakers are well known in the prior art. In contrast to so-called dynamic speakers where a membrane is moved by a coil in a magnet system, a membrane of a piezoelectric speaker is moved by a piezoelectric crystal. Piezoelectricity is the ability of certain crystals to generate a voltage in response to applied mechanical stress. The piezoelectric effect is reversible, meaning that piezoelectric crystals can change shape by a small amount when an external voltage is applied. The deformation is quite small, but sufficient to produce sound.
In the prior art two kinds of piezoelectric speakers are known: speakers having a excitation in a direction transverse to the plane of the membrane, that is to say in the direction of the sound emanation, and speakers having an excitation in a direction parallel to the plane of the membrane, that is to say transverse to the direction of the sound emanation. The first kind of piezoelectric speakers work in a similar way to dynamic speakers with a moving coil where the excitation area of the membrane, i.e. the area where force is induced into the membrane, performs a more or less translatory movement (in the following also referred as type A speaker). In contrast, the movement of a membrane of a piezoelectric speaker of the second kind, comprises no substantial translatory component, but substantially a bending component (in the following also referred as type B speaker). Consequently, the mechanical and hence the acoustic behavior of these two types is completely different, which is outlined hereinafter.
At this point a difference should be made between the excitation of a membrane and the movement caused thereby. Whereas the excitation of type A and type B speakers are transverse to one another, in both cases the membrane moves in a main direction causing the surrounding air to compress and decompress. Consequently, a sound wave is emitted in this main direction, which strictly speaking is the summation vector of sound vectors in the different directions. Normally, this main direction is simply the axis of the speaker. It should further be noted that applying a voltage to a piezoelectric crystal causes a dilatation/contraction in a main deformation direction. However, there is also a small deformation in the other axis, which for the sake of the invention is neglected. Finally, it should be noted that a substantial translatory component of the movement of the membrane does not exclude another moving component, in particular a bending component, and vice versa. However, a substantial translatory component/substantial bending component means that translation/bending prevails.
When a membrane of a type A speaker is excited, besides the translatory movement of the excitation area of the membrane there are also other components of movement of the remaining areas. Firstly, the area between the edge of the membrane, which is normally fixed to a housing, and the excitation area, moves according to the translatory movement of the excitation area relative to the fixed edge. Accordingly, said area performs a kind of rolling (compensation) movement, because of which it is generally much more compliant than the center area, which center area does not need to perform a compensation movement. Moreover, the so-called dome, which is the inside of the ring-shaped excitation area in case of common dynamic speakers, is bent upwards and downwards due to acceleration forces and pressure forces. However, there are also speakers with a more or less rigid plate serving as a membrane where said bending may be neglected. In any case, a membrane in addition tends to move according to its natural oscillation when it is excited. These oscillations are also known as standing waves or so-called modes. The frequency and amplitude of each mode depends on various parameters, such as shape and dimension of said membrane as well as material and thickness. This behavior and the consequences thereof are explained hereinafter with reference to the FIGS. 1 to 3:
FIG. 1 shows a cross section as well as a top view of a type A piezoelectric speaker 1′, which comprises a housing 2, a membrane 4 and a piezoelectric crystal 5′. The membrane 4 is connected to the housing 2 at the membrane's edges, e.g. by means of a glue. In the resulting space the piezoelectric crystal 5′ is attached between the housing 2 and the membrane 4. By applying a voltage across the piezoelectric crystal 5′, it dilates or contracts so that the membrane 4 is moved upwards (indicated with thin lines) or downwards in a main direction MD thus compressing or decompressing the air above the membrane 4 causing sound. To ease this movement, the membrane 4 comprises a corrugation at the outer section as can be seen in FIG. 1. This measure makes the membrane 4 softer at the outer section, that is to say increases the compliance. In contrast, the membrane 4 is stiffer in the center area. Hence, one will of course appreciate that the center/excitation area of the membrane 4 is moved mainly transitorily. Besides the translatory movement shown in FIG. 1 there are also further movements, e.g. the standing waves mentioned before.
FIG. 2 shows the movement of the membrane 4 (simply shown by a bold line) according to these standing waves or modes. On the left there is shown the first order mode, that is to say the bending of the membrane 4 according to its natural resonant frequency. Besides, there are harmonics. In FIG. 2 the first (center) and the second harmonic (right hand), that is to say, the second and third order modes are shown where the membrane 4 has one or two nodes respectively. The volume, which is shifted by the membrane 4 is visualized by a hatched area. One will easily appreciate that only the odd modes cause a substantial sound pressure since the sum of the hatched areas above and below the idle position of the membrane 4 is unequal to zero, whereas said sum in case of even modes causes substantially no sound.
FIG. 3 now shows the frequency response of the speaker 1′, taking into consideration the teachings of FIG. 2. On the abscissa the frequency f is shown, on the ordinate the sound pressure p. Every odd mode n=1, 3, . . . causes an elevation in the frequency response (due to the moved volume), every even mode n=2, 4, . . . a depression (no moved volume but dissipation of input power due to inner friction). It should be noted that the conditions are simplified in this graph and the graph is just for illustrating the general physical correlations. The frequency response of a real speaker may have a completely different frequency response.
However, this behavior of the speaker is not wanted as these elevations and depressions cause varying loudness at different frequencies. A number of methods have been found to damp these modes so as to decrease their influence so that the frequency response of a speaker gets as flat as possible. One method is to make the center area of the membrane sufficiently stiff so that natural modes only occur at higher frequencies. In this case often two materials are used, a rigid one for the center area and a soft one for the edge area. One further method is disclosed in GB1122698 where asymmetrical membranes are proposed, which are excited in the center of gravity. Yet another method is to shift the point of excitation of a symmetrical membrane away from the center of gravity, so that the disturbing modes are less excited. However, the frequency respectively the wavelength of the modes of the membrane 4 itself is not changed thereby. What is ideally left when designing a type A speaker is the so called “piston mode”, which is illustrated in FIG. 1 (got its name because the membrane in the center area moves like a piston, that is to say transitorily). It should be noted at this point that the piston mode should not be confused with the first order mode, which first order mode moves in the opposite direction to the piston mode.
Turning now to type B speakers, a completely different physics is presented. FIG. 4 shows the principle design of such a device in cross section as well as in a top view. The type B piezoelectric speaker 1 comprises a housing 2, a membrane 4 and a piezoelectric layer 5. The membrane 4 again is connected to the housing 2 at the membranes edges, e.g. by means of a glue. In contrast to a type A speaker, here the piezoelectric crystal exists in the form of a piezoelectric layer 5, which is attached to membrane 4 without touching the housing 2. Again, the piezoelectric crystal 5 dilates or contracts by applying a voltage so that the membrane 4 is moved upwards (indicated in thin lines) or downwards in a main direction MD. In contrast to type A speakers, the piezoelectric layer 5 dilates or contracts in a direction transverse to said main direction MD, that is to say in the plane of the membrane 4 in the present example. Therefore, the excitation area is not moved transitorily, but bent. However, also said bending compresses or decompresses the air above the membrane 4, causing sound. To ease this movement, the membrane 4 again comprises a corrugation at the outer section. This measure makes the membrane 4 softer at the outer section, that is to say, increases the compliance. In contrast to a type A speaker, the edge of the center/excitation area is not moved, but only turned. Again, there are standing waves besides the bending movement shown in FIG. 4.
The physics of the standing waves is to a large extent the same as for type A speakers so that a separate discussion is omitted for the sake of brevity. However, in contrast to a type A speaker, a type B speaker has to have odd modes n=1, 3, . . . . Otherwise, if they will be completely damped, there is no sound any more since there is no piston mode, which would generate sound.
Nevertheless, the type B speakers suffer from similar problems with respect to the frequency response, since here odd modes cause elevations and even modes cause depressions in the frequency response as well. Unfortunately, the teachings for type A speakers are not generally applicable to type B speakers. It is particularly impossible to apply the teachings of a rigid plate with a soft border area. One will easily understand that the bending of the membrane is essential for the function of the speaker. Therefore, a rigid membrane is a contradiction to a good efficiency of a type B speaker. Moreover, it is particularly impossible to apply the teachings with respect to shifting the point of excitation as mentioned above. Whereas the excitation area of type A speakers is comparatively small, that is to say 5% of the total membrane area, the excitation area of type B speakers is comparatively large, that is to say 20% of the total membrane area and more. One skilled in the art of course will understand that for the function of a type A speaker the dimension of the excitation area is more or less irrelevant, assuming that the membrane is sufficiently rigid in the center area. Accordingly, it is also clear that a type B speaker cannot be excited at a single point, but has to be excited in a sufficiently large area. Normally, the excitation area of a type B speaker is equivalent to the area of the piezoelectric layer. Only if the piezoelectric layer is partly attached to the speaker housing, for instance if the whole membrane comprises a piezoelectric layer because of easier manufacturing, those parts do not contribute to the excitation area. Finally, the first order mode of a type A speaker and a type B speaker show a completely different behavior. In a type A speaker the first order mode moves in the opposite direction to the piston mode, which means that the first order mode reduces the loudness of a type A speaker. In contrast, the first order mode is the one that (mainly) produces the sound of a type B speaker. One will of course understand that a designer of a type A speaker aims to get rid of the bending modes. In particular, he will try to avoid the influence of the first order mode as much as possible as this one has the greatest impact on loudness reduction mentioned above. However, if a designer tries also to avoid the first order mode when designing a type B speaker, he will of course fail to make a speaker of sufficient performance.
Hence, it is an object of the invention, to provide a type B speaker that has a substantially flat frequency response and design rules therefor.